1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an image forming apparatus of the type including at least one developer carrier configured to carry a high viscosity, high density developing liquid, which consists of a carrier liquid and toner dispersed therein, and developing a latent image formed on an image carrier with the developer carrier deposited on the developer carrier.
2. Description of the Background Art
Japanese Patent Laid-Open Publication No. 7-239615 and Japanese Patent Application No. 11-38447, for example, each discloses an image forming system including a developer carrier formed with an elastic layer thereon and held in contact with an image carrier to form a nip. A developing liquid consisting of a carrier liquid and toner dispersed therein is deposited on the developer carrier in the form of a thin layer. The carrier liquid and toner in the thin layer are electrostatically transferred to a latent image formed on the image carrier at the nip.
In the image forming system described above, toner grains deposit on the latent image of the image carrier at the nip while, at the same time, the carrier liquid deposited on the carrier grains also moves toward the image carrier. This brings about a problem that not only the toner grains but also the excess carrier liquid deposit on the latent image, aggravating the consumption of the carrier liquid. Moreover, the excess carrier liquid increases the running cost of the system, and its amount effects the fixation of the toner on a sheet.
As for the background or non-image portion of the image carrier, it is a common practice to transfer some carrier liquid to the background at the nip while preventing the toner from depositing on the background. When the toner is deposited on the background, it is caused to move toward the developer carrier and removed thereby within the nip. However, the toner is apt to deposit on the background of the image carrier in spite of such an expedient and remain on the image carrier even after the image carrier has moved away from the nip, constituting residual toner.
To obviate residual toner, it has been customary to form a strong electric field between the background of the image carrier and the developer carrier (background electric field hereinafter), thereby preventing the toner from depositing on the background. The background electric field obviates toner deposition on the background more positively as it becomes stronger. For the same purpose, Japanese Patent Application No. 2000-42582 proposes to use a removing member and forms an electric field between the background and the removing member (removal electric field hereinafter). The removal electric field attracts floating residual toner toward the removing member away from the image carrier, thereby protecting a toner image from fog ascribable to the residual toner.
The problem with the background electric field is that when it is intensified, a force pressing the residual toner in the non-image portion against the developer carrier grows stronger. It even occurs that the background electric field is excessively intensified for the purpose of obviating toner deposition on the background, causing the toner pressed against the developer carrier to cohere. This is also true with the removal electric field scheme; that is, the stronger the removal electric field, the more the residual toner attracted toward the removing member coheres. The cohered toner has a grain size larger than the original grain size and cannot faithfully reproduce thin lines when reused for development. It is therefore desirable to prevent the residual toner from cohering.
In the image forming apparatus of the type described, to transfer the toner image from the image carrier to a sheet, an image transfer roller causes the sheet to contact the toner image on the image carrier while a bias opposite in polarity to the toner image is applied to the image transfer roller. At this instant, assume that the developer layer formed on the image carrier is excessively thick, i.e., the amount of the carrier liquid or that of the toner is excessive. Then, even when the sheet is brought into contact with the surface of the image carrier, the developer carrier and sheet often fail to closely contact each other, resulting in a short toner transfer ratio, the blurring of an image or the thickening of characters. Moreover, carrier liquid consumption is aggravated and increases the running cost. On the other hand, if the amount of the carrier liquid is short, then image transfer using electrophoresis is obstructed with the result that image density is lowered over the entire image or in part of an image corresponding to the recesses of the irregular surface of a sheet or the entire image.
It has been proposed to leave an adequate amount of carrier liquid that does not bring about the problems described above, and sweep the excessive carrier liquid with a sweep roller or similar excess liquid removing means.
Today, various kinds of sheets are available as a recording medium applicable to an image forming apparatus of the type described. As for full-color image formation, in particular, the application of a coated sheet covered with a coating layer for enhancing whiteness and smoothness is in study. If process conditions for image formation are fixedly applied to all of various kinds of sheets, then the problems stated above are likely to become more conspicuous, depending on the kind of sheets.
More specifically, assume that use is made of a sheet absorbing the carrier liquid little, a sheet having a smooth surface or a sheet coated with a relatively large amount of coating material, and that the conventional fixed process conditions assigned to plain copy sheets having a rough surface and easily absorbs the carrier liquid each. Then, the thickening of characters and the blurring of the trailing edge of a solid image are conspicuous, as determined by experiments. When some of the process conditions are varied to free an image from the above defects, other problems occur when use is made of a sheet easily absorbing the carrier liquid, a sheet having a rough surface or a sheet coated with a relatively small amount of coating material, as also determined by experiments. Fore example, the resulting image is low in image density over its entire area or in portions corresponding to the recesses of the irregular surface of a sheet or is practically lost in such portions.
To cope with various kinds of sheet, Japanese Patent Laid-Open Publication No. 8-297418, fire example, disposes a liquid film control system using excess liquid removing means whose liquid removing force is variable and switching the liquid removing force in accordance with the property of a sheet. The variable liquid removing force controls the thickness of a liquid film. The excess liquid removing means is implemented as a squeeze roller or a slit nozzle. The squeeze roller is positioned to face the surface of an image carrier at a preselected distance and rotatable in the same direction as the image carrier. The slit nozzle is also positioned to face the surface of the image carrier at a preselected distance and sends compressed air toward the image carrier. Such a liquid film control system is effective when use is made of low viscosity, low density developing liquid, e.g., a developing liquid with viscosity of about 1 mPa·s and consisting of an insulative carrier liquid Isopar (trade name) available from Exxon and 1 wt % to 2 wt % of toner.
Recently, replacing the conventional low viscosity, low density developing liquid with a high viscosity, high density developing liquid has been proposed. A developing liquid with high viscosity and high density has viscosity of about 50 mPa.S to 10,000 mPa.s and consisting of silicone oil, normal paraffin, Isopar M (trade name) also available from Exxon, vegetable oil, mineral oil or similar carrier liquid and 5 wt % to 40 wt % of toner. The liquid film control method stated earlier cannot easily control the film of such a developing liquid that is highly viscous and deposits on the image carrier only in a small amount. For example, compressed air sent from the slit nozzle cannot easily remove the developing liquid due to high viscosity. Further, because the highly dense developing liquid is left on the image carrier in the form of a thin film after development, it is difficult to cause the squeeze roller spaced from the image carrier to contact the carrier liquid layer on the image carrier for mechanical accuracy reasons.